Along with a greater demand for high fuel economy tires, improvements in the fuel economy of the tread, which constitutes a large part of a tire, and of other tire components such as the sidewall or base tread have recently been demanded. Known approaches to improve the fuel economy include the use of low reinforcing filler, the use of a smaller amount of filler, or the use of silica filler. However, since these approaches reduce reinforcing properties, it is generally difficult to simultaneously ensure fuel economy and reinforcing properties.
In view of fuel economy and environmental concerns, natural rubber is widely used. Natural rubber has a higher Mooney viscosity than other synthetic rubbers and is poor in processability. Usually, natural rubber is mixed with a peptizer and then masticated to reduce the Mooney viscosity before use, which results in poor productivity. Further, the mastication breaks the molecular chains of natural rubber and the resultant natural rubber unfortunately loses high-molecular-weight polymer characteristics that natural rubber originally has, such as good abrasion resistance, good fuel economy, and high rubber strength.
A report teaches that the processability of natural rubber can be improved by removing proteins from latex. For example, a method of aging with a proteolytic enzyme and a surfactant, a method of immersing a solid natural rubber swollen with a solvent in an alkali hydroxide, a method of adding a phosphate and then removing magnesium phosphate, and a method of washing with a surfactant are disclosed (Patent Literatures 1 to 5). Unfortunately, these methods fail to sufficiently remove proteins and hardly remove phospholipids and the like. Accordingly, there still remains room for improvement in fuel economy.
Another problem is that, although common natural rubber is not degraded under aging conditions at 80° C. for about 18 hours, degradation is observed when modified natural rubber from which proteins and the like have been highly removed is exposed to the same conditions, indicating that the modified natural rubber is inferior in heat aging resistance. As described above, it is generally difficult to achieve a balanced improvement in the fuel economy, reinforcing properties, and heat aging resistance of tires. Thus, improvements are desired. Furthermore, improvements in grip performance (wet-grip performance, performance on snow and ice), handling stability and the like are constantly needed for safety reasons.